The present invention relates to an optical device such as an optical isolator or an optical modulator, and more particularly to improvement in a polarization-independent optical isolator provided with fiber collimators on both sides of a non-reciprocal unit using tapered (wedge-type) birefringent plates.
The present invention provides a non-polarization-mode-dispersing optical isolator adapted to eliminate the difference in velocity between the ordinary ray and extraordinary ray occurring on tapered birefringent plates, by inserting a parallel-surfaced birefringent plate between a non-reciprocal unit and one fiber collimator in a polarization-independent isolator. This optical isolator is useful for, especially, the long distance, high speed communication.
As generally known, an optical isolator is a non-reciprocal optical device having a function of allowing the passage of light in one direction and preventing the passage thereof in the opposite direction, and used in large quantities for shutting off the reflected return light sent out from an optical part in an optical fiber communication system.
An optical fiber communication system used practically at present employs a system for sending out information from an output side with the intensity of light modulated, and detecting at a reception side the intensity of an optical signal by a direct light detecting method and then demodulating the information. Regarding the long distance communication using, for example, a submarine cable, an attempt has been made for transmitting a directly amplified optical signal by utilizing a plurality of optical fiber amplifiers incorporated in the intermediate portions of an optical cable. When such a method is employed, a number of optical isolators are required.
Various types of optical isolators have been developed. The optical isolators suitably incorporated in the above-mentioned optical cable include an optical isolator of the type which is provided with incident side and outgoing side fiber collimators on both sides of a non-reciprocal unit and does not depend on a plane of polarization as disclosed in Japanese Patent Publication No. 61-58809/1986 and corresponding U.S. Pat. No. 14,548,478. The non-reciprocal unit referred to above has a unitary combination of a 45.degree. Faraday rotator, and two tapered birefringent plates sandwiching the rotator therebetween. These tapered birefringent plates are arranged with the optical axes thereof staggered from each other at 45.degree..
In the forward direction, the parallel input rays from the incident side fiber collimator are separated into ordinary rays and extraordinary rays by a first tapered birefringent plate in a non-reciprocal unit, and a plane of polarization is turned in a 45-degree arc by a Faraday rotator, these two kinds of rays being turned into parallel rays by a second tapered birefringent plate, which parallel rays therefore enter an outgoing side fiber collimator. In the opposite direction, the reflected return light is also separated into ordinary rays and extraordinary rays by the second tapered birefringent plate, and a plane of polarization is turned in a -45.degree. arc due to the non-reciprocity of the Faraday rotator. Out of the separated rays, the ordinary rays are converted into extraordinary rays, and the extraordinary rays into ordinary rays, by the first tapered birefringent plate. Therefore, the separated rays which have passed through the first tapered birefringent plate spread and do not become parallel rays, so that none of these rays enters the incident side fiber collimator. Thus, the passage of light in one (forward) direction is allowed but that of light in the opposite direction is prevented.
In the transmission of an optical signal in the forward direction by the optical isolator described above, the incident light is separated into ordinary rays and extraordinary rays in the non-reciprocal unit. Since the velocity of the respective rays is different, a transfer lag is about 0.85 picoseconds in terms of time difference when tapered birefringent plates consisting, for example, of monocrystalline rutile and having a thickness of their optical path passing portions of about 0.5 mm are used. In the outgoing side fiber collimator, the ordinary rays and extraordinary rays are synthesized, and, therefore, disorder (increase in the width of optical pulse) corresponding to the transfer lag occurs in the waveform of an optical signal.
The transfer lag of an optical signal caused by the difference in velocity (polarization mode dispersion) between the ordinary rays and extraordinary rays is relatively small in a single optical isolator. In fact, this transfer lag is negligible and it poses little problem at an optical communication speed of around 2.5 Gbit/s currently employed. However, since a great number of (about 50-150) optical isolators are incorporated in a long distance optical communication (optical communication using, for example, a submarine cable) system as described above, the turbulence of the waveforms of optical signals becomes large, so that, when such an optical system is operated at a high speed of for example, 10 Gbit/s, information transmission normally becomes difficult.